Confocal techniques are known for measuring distances. As described in U.S. Pat. No. 5,785,651, it is known to utilize confocal microscopy as an effective means of obtaining high resolution images in both lateral dimension and in depth. In typical confocal microscopy, a monochromatic point source of light is projected onto a surface and a portion of the reflected light is separated and then imaged onto a detector pinhole. The amount of light through the pinhole is measured by a detector. The intensity of the light at the pinhole is diminished for out of focus targets. Therefore, the total light energy reaching the detector is inversely related to the distance a target is from the plane of best focus. This signal can be used to control the position of the microscope until a maximum signal is returned. The position of the microscope can then be recorded and correlated to the height of the object.
It is also known to use chromatic confocal techniques in optical height sensors. As described in U.S. Patent Application Publication No. US2006/0109483 A1, which is hereby incorporated herein by reference in its entirety, an optical element having axial chromatic aberration, also referred to as axial or longitudinal chromatic dispersion, may be used to focus a broadband light source such that the axial distance to the focus varies with the wavelength. Thus, only one wavelength will be precisely focused on a surface, and the height of the surface determines which wavelength is best focused. Upon reflection from the surface, the light is refocused onto a small detector aperture, such as a pinhole or the end of an optical fiber. In one embodiment, a 50 micron core fiber is utilized as both the source and receiver pinhole (the end of the fiber acting as the pinhole). Upon reflection from a surface and passing back through the optical system to the in/out fiber, only the wavelength that is well-focused on the surface is well-focused on the fiber. All of the other wavelengths are poorly focused on the fiber, and so will not couple much power into the fiber. Therefore, the signal level will be greatest for the wavelength corresponding to the height of the object. A spectrometer at the detector measures the signal level for each wavelength, which effectively indicates the height of the object.
Certain manufacturers refer to a practical and compact optical assembly that is suitable for chromatic confocal ranging in an industrial setting as a chromatic point sensor (CPS) and/or as an “optical pen”. In such CPS optical pens, the standard design approach has been to use the end of the in/out fiber as the detector aperture. To provide a smaller detector aperture, which generally increases the measurement resolution, the standard technique is to taper the diameter of the fiber and core at its end. For example, certain manufacturers are known to have tapered the end of a 50 micron diameter core fiber to a 20 micron diameter core by a heating and stretching process (e.g. see J. Villatoro et al., “Fabrication and modeling of uniform-waist single mode tapered optical fiber sensors,” Appl. Opt., 42, 2278-2283 (2003)). However, a number of problems or deficiencies have emerged in relation to existing CPS optical pen assemblies that are available for chromatic confocal range sensing. For example, one major problem is that, in order to maintain their specified performance the existing types of CPS optical pen assemblies generally require recalibration and/or factory servicing if the optical fiber needs to be replaced. This is both inconvenient and expensive for users.
The present invention is directed to providing an apparatus that overcomes the foregoing and other disadvantages. More specifically, an optical fiber interface configuration is provided for a CPS optical pen which increases resolution without requiring a tapered fiber, and provides other advantages.